OK. We all know the basics of how new species get created. They keep mutating into different stuff, until the original and old one are no longer compatible. This is a very slow process, taking thousands or tens of thousands of years and many generations. Now, this all works well with simple stuff that reproduces asexually.

But move this to sexual reproduction and there's a problem.

How did sexes get created in the first place?

Lets assume we have the first male or female that is the result of mutation. Now, mutations like this would be quite rare. So that means that the first male has to find a first female (also the product of mutation). Now given the rarity and slow speed of this sort of mutation, it seems highly unlikely this would ever happen.

To make matters worse, you need a suitable gene pool, so one male and one female isn't going to be enough. You need to somehow get a whole small community. But again, mutation is a slow process, so highly unlikely you'll get that many mutations in one place.

The same would occur when two species evolve to the point of no longer being compatible. So as apes evolve into men, was there a point where a caveman could mate with a female ape? If not, how was it that there were enough caveman mutations to produce a new species?

It just seems like current theories of evolution don't really explain this at all.

To summarize, species that reproduce sexuality are less likely to pass on diseases or specific vulnerabilties do diseases, and they preserve a wider variety of genetic potential by passing on more genetic traits than just what one parent or the other needed to survive.

As far as why sexual differentation (?) appeared in the first place, aside from random mutation, off the top of my head I'd guess it came from species that developed communication via chemical signals as a survival tool. If you think about it, sexual reproduction boils down to one branch of a species transmitting/receiving chemicals to/from another branch of the species who can't produce the same chemicals. From then on it's just a matter of one branch specializing in gestation with other branch being able to develop without the ability to gestate.

Keep in mind, that doesn't mean that gestation is exclusively male or female, some seahorses are famous for the males gestating thier young, and female fish can lay eggs so that males can inseminate them and the eggs can gestate outside of either parent's body. A species with sexual reproduction is just more likely to have female members whose biology needs to make room for gestation (usually accompanied by maternal behaviors/traits) and male members who don't have to develop this trait.

One could argue that this naturally limits a woman's ability to do anything other than give birth and nurture children, but humans moved beyond that as soon as we started living in communities, if not before, and increased our average lifespans to the point that there were usually enough grandparents around to care for children. And that sort of isolation of females is hardly a universal for other species - a majority of females from lobsters to birds to wolves to gorillas are able to survive and even feed thier offspring without the presence of a male.

There are actually creatures that are alive today that use earlier versions of sexual reproduction. In addition to the bacteria which exchange DNA randomly before splitting, there are creatures which are both "male" and "female".

When sexual reproduction occurs, both creatures provide genetic material and originally both creatures provide structural ovum as well. For macroscopic organisms, specialized organs are pretty much required for this, and thus genitalia are born. Still, the older organisms have both sets of genitalia.

Once that system is in place, a creature can lose one copy of the genitals and still be able to produce a new generation at all - which is of course the only thing that evolution actually checks for. But if it was for some reason important for there to be an actual advantage for something to propagate - it's totally there.

Imagine that you're in a situation where you have both sets of genitalia, as all original macroscopic organisms originally were. Now you disable your own "female" organs, preventing you from carrying children. But it doesn't prevent you from having sex and producing children in other creatures of your type. This means that you suddenly don't have to spend all the massive amount of energy to produce ova and raise babies. Once you mate, you're actually free to mate again because you don't have any babies growing inside you. Every time you mate you're producing half as many offspring, but you personally can mate three times as many times or twenty times, the sky and the harem is the limit, because you don't have any ova of your own.

Is this disadvanageous to the overall species? Yeah, it probably is. But from a purely "selfish gene" point of view the male is actually pretty easy to understand - it's a net advantage to the Y chromosome's ability to spread for it to exist. And once the male parasite spreads into the species, there's no selective pressure at all for the remaining normal types to retain their male genitalia.

But remember: evolution does not require that something be an advantage to the species or even the individual. It only requires that things exist at all. Evolution is a tautology. Things that don't reproduce don't reproduce. So something can be a straight up disadvantage and still spread through the population so long as it isn't 100% lethal.

Actually it's not really a question of male/female. The problem is two fold, the first is Egg/sperm generation which actually is hormonally derived from stem cells. The second is the introduction of the Y chromosone into the male chromosone pair grouping which results in gender differences in advanced species.

Even when you get the one reproduction organ that doesn't mean one can't change. Some creatues change their gender as they age, first being one gender and then the other. Generally being aquatic most of the major reproduction stuff takes place in the open water.

In modern mamels there is only one reproduction system. Pull the hormonal switches one way you get a male and the other way you get a female. Some organisms (like plants) have multiple reproduction systems and almost always have male/female functionality both active at the same time.

Well, not every species derives male/female from Y chromosomes. In bees for example, the males are parthenogenicized from the mother and have only one set of chromosomes, while the workers and prospective queens are created sexually from all of a drone's DNA and half the genetic sample of the queen. But pretty much, yeah.

There's more than one way to have life. It was 'proven' to be useful to Eukarya, and so passed down to us in many forms.

There are other types of life, and some have other types of 'sex' - fungi for instance can have hundreds of 'sexes' in an individual.

It's a mathmatical advantage to have reproduction by the sharing rather than copying or splitting of genetic data... As soon as you have macroscopic creatures, multicelled creatures, you're going to have specialized cells doing specific jobs. And one specific job is reproduction.

Hormone switches are merely one of the many construction flags in a multi-celled organism. In mammals, there's no real structural difference between males and females, and one could become the other with the right flags. It just happens to be coded into the genes a layer of 'defaulting' as well...

...In other animals, many other things send up these flags. Exertion, temperature, even sexual position can be used to temporarily or permenently assign the sex of an individual.

Current theories of evolution don't care what happened, it only explains the overall mechanics for it. Evolution doesn't hinge on there even being sexes, let alone ones triggered by genetic or external forces.

It's kinda like how the concept 'a map' doesn't really care if it's a map of a castle or roads or hills underwater or geranium deposits, it's still a map.

The origins of sex is still considered an open question in evolutionary biology, notably due to the two-fold cost of sex first denoted by John Maynard-Smith.

My favorite solution to the problem is called the Red Queen hypothesis, where sex is used to maximize recombination of genes in arms races against parasites. One of the key pieces of evidence for that theory is the extinction rate of asexual or parthenogenetic eukaryotes is much higher than for sexual ones (Bdelloid rotifers being the sole exception as noted by Dawkins in The Ancestor's Tale).

One study was done in fish pools, where the same species of fish could either be parthenogenetic or sexual (the research was featured in PBS's Evolution documentary) but it was inconclusive.

My second favorite theory is called Muller's Ratchet. Basically, due to mutation, deleterious alleles accumulate over time in non sexual populations. Sex can recombine these alleles out of a population.

Whatever the reason, there's one thing biologists all over the world can agree on: It's a lot of fun.

Reading the original post again, i notice that we're not really answering the question. First RC, let's deal with your questions on speciation:

RC said:"So as apes evolve into men, was there a point where a caveman could mate with a female ape? If not, how was it that there were enough caveman mutations to produce a new species?"

The dominant view is that the model that best explains this is the Dobzhansky-Muller model. Basically, this is an explanation of how molecular changes occur that can lead to reproductive barriers in allopatric populations. Allopatry means that the population is separated by a physical geographic barrier.

In our case, we can imagine an ape-like ancestor to both humans and chimps. An event that separates out two populations occur. One stays in its forest home (destined to become chimps over time) and one leaves (our ancestors). At the start, if one member of the population comes back the original population, it can still breed.

BUT, let's say we have a mutation event. Instead of aa/bb, the savannah population now has alleles Aa/bb. The forest population has a different mutational event, aa/bB. At this point, the populations can still come back together, mate and produce viable offpring some of the time. But, if the geographic barrier remains, and the new mutations proved beneficial and become fixed (result AAbb vs. aaBB), then breeding leads all offspring having the genotype AaBb.

Because the big A and big B alleles have never "seen" each other before, they can be incompatible, leading to developmental failure. This process has been shown in labs with drosphila and other model organisms and is very powerful: even a few generations apart can cause allopatric populations to have low numbers of hybrids and even complete hybrid mortality.

After that happens, it is simply a matter of natural selection reinforcing barriers to copulation between the two populations (known as premating isolation in the biology lingo). They are then free to inhabit the same geographic area as different species, even though they may look very similar at this stage.

One really cool thing is how disadvantageous it apparently is for a gene to be sex-determining. Sex-determining genes have a higher turnover rate than just about any other type. So much so that sex genes are basically useless for determining how related to species are.

Oh ya, and haploid males really seems like the best way to go in a non-monogamous species. It's kinda surprising that hymenopterae are the only animals to take advantage of it.

Reading the original post again, i notice that we're not really answering the question. First RC, let's deal with your questions on speciation:

RC said:"So as apes evolve into men, was there a point where a caveman could mate with a female ape? If not, how was it that there were enough caveman mutations to produce a new species?"

I don't think anyone was deliberately ignoring this question, it just seemed that addressing the question of where sexes came from in the first place was more important. Basically, we can accept that sexual reproduction didn't interfere with humans seperating from apes as a species, because we know that sexual reproduction happened much, much earlier on the evolutionarly ladder (early multi-cellular life forms) and it didn't stop the branching of new species from old species.

Getting straight to the point: yes, according to general evolutionary theory, there was a point where cavemen/women could and were reproducing with apes. Spend a weekend hanging in bars near a "party" college, it won't seem that far-fetched